At the Ol Pejeta Conservancy, a wildlife preserve in central Kenya, lions and cheetahs mingle with zebras and elephants across many miles of savannah – grasslands with "whistling thorn" acacia trees dotting the landscape here and there. Twenty years ago, the savanna was littered with them. Then came invasive big-headed ants that killed native ants — and left the acacia trees vulnerable. Over time, elephants have knocked down many of the trees. That has altered the landscape — and the diets of other animals in the local food web.

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